Conventionally, a gas sensor is used for determining the concentration of a particular component (oxygen, etc.) in exhaust gas of an internal combustion engine. The gas sensor has a gas sensor element provided therein. The gas sensor element has, on a forward end side thereof, a detection element including a solid electrolyte body and a pair of electrodes disposed on the solid electrolyte body. The gas sensor element may be exposed to poisoning substances (e.g., silicon and phosphorus) contained in exhaust gas and may suffer adhesion of water droplets contained in exhaust gas. Thus, in order to trap poisoning substances and to prevent direct contact of water droplets with the gas sensor element, the outer surface of the gas sensor element is covered with a porous protection layer. That is, the detection element, which is exposed to a gas-to-be-measured (exhaust gas), is entirely covered with the porous protection layer.
Also, the following technique has been developed: the porous protection layer is formed of two layers (i.e., an upper layer and a lower layer), and, by means of the lower layer having porosity higher than that of the upper layer, an anchor effect brings the upper layer into close contact with the lower layer, whose surface is roughened by pores (see Patent Documents 1 and 2).